1. Field of the Invention
The present invention relates to a device and a method for efficient outcoupling of optical power in an external cavity laser, such that the outcoupled light contains a reduced fraction of spontaneous emission compared with traditional devices and methods.
2. Background Information
An external cavity laser is a type of laser, which is often used when it is desirable to be able to vary the wavelength of the light emitted from the laser. An example of an external cavity laser is shown in FIG. 1a. It comprises a light emitting and/or amplifying element 100, for example a semiconductor laser die, a first reflecting external element 170 and a second reflecting external element 150. The term xe2x80x9ccavityxe2x80x9d refers to an optical resonator cavity, which is the space between the end reflecting elements 150, 170 in a laser. The term xe2x80x9cexternalxe2x80x9d refers to that the cavity is longer than the light emitting and/or amplifying element 100. The first reflecting external element 170 can be replaced with the facet 102 of the light emitting and/or amplifying element 100, if said facet 102 is at least partly reflecting. The second reflecting element is often arranged in combination with a wavelength selective element, for example a diffraction grating 140. Such configuration with a reflecting element 150 and a diffraction grating 140 is often referred to as a Littman cavity. In the Littman cavity, the laser wavelength can be varied by changing the angle of the reflecting element 150 relative the diffraction grating 140. FIG. 1b shows a cavity configuration where the diffraction grating 141 itself is the second reflecting element. Such configuration is often referred to as a Littrow cavity. In the Littrow cavity, the laser wavelength can be varied by changing the angle of the grating relative the optical axis 199 in the cavity.
If, but not only if, the light emitting and/or amplifying element 100 is a semiconductor laser die, said light emitting and/or amplifying element includes an optical waveguide 106. The optical waveguide 106 is narrower than the optical beam 181 and at least one converging optical element 110 is used for collimating the diverging beam 180 and focusing the collimated beam 189. If a first reflecting external element 170 is used, at least one converging optical element 160 is used for collimating the diverging beam 191 and focusing the collimated beam 193.
All interfaces, except for the first and second reflecting elements, should be arranged such that said interfaces do not reflect the light in the direction of the cavity optical axis 199. Alternatively, said interfaces can be coated for anti-reflection. If, but not only if, the light emitting and/or amplifying element 100 is a semiconductor laser die, the facet 104, facing the direction of the second reflecting element, is often coated for anti-reflection.
The optical power can be coupled out of the cavity, to the output beam or optical fiber, in several ways. For example, if a diffraction grating is used in a Littman or Littrow configuration, the light not diffracted but reflected from the diffraction grating, can be used as output optical power 184. If the first reflecting element is a partly reflecting facet 102 of the light emitting and/or amplifying element 100, the power 191 transmitted through the facet 102 can be used as the external cavity laser output. These, but not limited to these, examples of outcoupling methods will be referred to as traditional outcoupling methods.
The coherent emission from the external cavity laser is typically spectrally very narrow. However, the light emitting and/or amplifying element 100 also generates a broad spectrum of spontaneous emission. For a traditional external cavity laser emitting a total of, for example, 1 mW optical power into a single mode fiber, approximately 10 xcexcW of the power is spontaneous emission. This power ratio of 20 dB is insufficient for many applications, for example, when the laser is used for characterization of optical filters. A laser source emitting a smaller fraction of spontaneous emission would be very attractive.
A method and device for reducing the fraction of spontaneous emission in the optical output from external cavity lasers has been demonstrated by Edgar Leckel et al. [Ref. 1]. The demonstrated device was used in a Littman external cavity laser as shown in FIG. 2. A beam-splitter 220 was placed between the wavelength selective element 240, in this case a diffraction grating, and the light emitting and/or amplifying element 200, in this case a semiconductor laser die. The beam-splitter 220 deflects a fraction of the incident lights in two opposite directions 224226 corresponding to the two directions of propagating light 281288 inside the cavity. The outcoupled beam 224, originating from the light 281 propagating from the semiconductor laser die 200 towards the diffraction grating 240, contains the same fraction spontaneous emission as for traditional outcoupling. The outcoupled beam 226, originating from the light 288 propagating from the diffraction grating 240 towards the semiconductor laser die 200, is spectrally filtered, such that the spontaneous emission has an angular distribution around the direction of propagation for the lasing wavelength. If the spectrally filtered outcoupled beam 226 is also spatially filtered, for example using a single mode optical fiber, the fraction of spontaneous emission of said beam is typically reduced by a factor of 1000. The main disadvantage with this method is that a large amount of the total outcoupled optical power is not spectrally filtered.
The optical power in the spectrally filtered beam 226 can be no more than equal to the optical power in the beam 224 that is not spectrally filtered. Therefore, no more than xc2xd of the optical power outcoupled by the beam-splitter can be used as a low spontaneous emission light source.
In U.S. Pat. No. 5,406,571 is a tunable laser oscillator is disclosed, which comprises a laser medium, an optical resonator, a wavelength selective element for adjusting the wavelength of a laser beam, and optical means for broadening the radiation in the resonator. The laser beam is decoupled from the resonator by means of an optical element after having passed the broadening means and prior to passing again through the laser medium. The laser beam is decoupled from the resonator such that its direction is independent of the beam wavelength.
The laser beam generated in an optical amplifying medium is divided into two beams by means of a prism. One of the beams comprises a reflection from the prism""s first surface. No reduction of the spontaneous emission is obtained in this beam. The other beam consists of diffracted beam inside the prism. Thus, the beam is broadened and illuminates a larger area of the wavelength-detecting element. The beam is diffracted so that it propagates in same beam path but in opposed direction. The light is finally decoupled out of the laser cavity by means of the first prism. The first prism is realized in two geometries and a number of cavity configurations. However, the object of laser according to this document is:
to achieve high spectral purity, i.e. low spontaneous emission, for one of the beams decoupled from the cavity,
that high spectral purity at one of the decoupled beams is achieved without any major structural changes in the structural changes in the laser cavity,
that the direction of the decoupled spectrally pure beam is independent of the wavelength as well as the position of the wavelength selective element.
Moreover, this document does not mention or gives any hint of using a Faraday rotator.
However, a retardation plate is mentioned, which is a completely different element.
The present invention can couple part of a light beam propagating from a wavelength selective element, towards a light emitting and/or amplifying element, out of an optical cavity, without any outcoupling of the beam propagating from the light emitting and amplifying element towards the diffraction grating.
The present invention can solve problems of the prior art by arranging the initially mentioned elements and adding polarization selectivity to the beam-splitting optical element, and the introduction of a Faraday-rotator element. A polarization selective beam-splitting optical element is an element that essentially fully transmits, without deflection, incident light of one polarization and essentially fully deflects the light of the orthogonal polarization.
The external cavity laser and outcoupling device elements can be arranged such that linearly polarized light is essentially fully transmitted through the polarization selective beam-splitting optical element when propagating in a first direction, and is incident on the wavelength selective feedback element with a polarization such that said selective feedback element has essentially optimum efficiency, and is at least partly outcoupled from the cavity by the polarization selective beam-splitting optical element when propagating in a second direction. The light emitting and/or amplifying element is a semiconductor laser die and includes a narrow waveguide. It is also possible to use light converging elements. The device may also comprise a first reflecting element. In one embodiment, the wavelength selective feedback element redirects the light towards a retroreflector. A light beam path from said at least one light emitting and/or amplifying element to said feedback element is substantially L-shaped.
The invention also relates to a method for outcoupling of light in an external cavity laser, which external cavity laser comprises at least one light emitting and/or amplifying element, at least one wavelength selective feedback element. Thus, the method can include the steps of utilizing at least one polarization selective beam-splitting optical element, and at least one Faraday-rotator element, arranging said external cavity laser and outcoupling elements such that linearly polarized light is essentially fully transmitted through the polarization selective beam-splitting optical element when propagating in a first direction, and is incident on the wavelength selective feedback element with a polarization such that said selective feedback element has essentially optimum efficiency, and is at least partly outcoupled from the cavity by the polarization selective beam-splitting optical element when propagating in a second direction.
The invention also relates to a method for outcoupling of light in an external cavity laser, which external cavity laser comprises at least one light emitting and/or amplifying element, at least one wavelength selective feedback element. The method can include the steps of generating light of essentially linear polarization in said light emitting and/or amplifying element and emitting said light in a diverging beam, collimating said diverging beam in a light converging optical element to an essentially linearly polarized light beam, essentially fully transmitting said essentially linearly polarized light beam through a polarization selective beam-splitting optical element in a first direction, rotating the plane of polarization of said light beam in an angle xcex1+m180xc2x0, wherein m is an integer 0,1, 2, 3, etc, redirecting said light beam with rotated plane of polarization, rotating the plane of polarization of said redirected light beam so that the angle of the light polarization becomes an angle 2xcex1 from a preferred angle of transmission through said polarization selective beam-splitting optical element, partly transmitting and partly outcoupling said redirected light beam when propagating through said polarization selective beam-splitting optical element in a second direction. Moreover, the fraction of light transmitted and outcoupled in the polarization selective beam-splitting optical element, when propagating in the second direction, is essentially cos2(2xcex1) and essentially sin2(2xcex1), respectively. The method can include the further step of selecting an appropriate angle a for determining the fraction of light coupled out of the cavity.